RFID systems include RFID transponders and read/write units (R/W-unit) for writing data to the RFID transponder (downlink) or for reading data from a RFID transponder (uplink). The RFID transponder front end has a high quality inductor, which is used as antenna and a resonant capacitor coupled to the antenna. The inductive antenna and the resonant capacitor form a parallel-resonant circuit, which is excited by an external RF signal from the R/W-unit during downlink. For passive RFID transponders, the received RF signal is rectified and converted into an internal supply voltage for supplying the RFID transponder. The downlink data transmission is typically performed by a 100% amplitude shift keying modulation. According to this modulation, the R/W-unit sends bursts of RF signals and the RFID transponder determines based on the length of the bursts and pauses between the bursts the data to be received. For uplink data transmission, many RFID transponders use frequency shift keying (FSK), for example 134.3 kHz for a low bit and 123.2 kHz for a high bit.
Transponders with only one antenna are sensitive to orientation. Therefore, advanced transponders are provided with three antennas in the form of three LC resonant circuits which are arranged in a three-dimensional configuration. The three antenna circuits have antenna structures that are physically oriented at mutually 90 degrees. With such a transponder, signals from a transceiver/interrogator placed, for example, in a vehicle are detected independently of orientation in space of the transponder.
FIG. 1 shows an RFID system comprising an R/W-unit also referred to as interrogator 10 which in the case of a passive entry system may be located in a vehicle. The interrogator comprises for example a control unit 10a, an LF transceiver 10b and a UHF receiver 10c. The RFID system further comprises an identification device or key or transponder 12 comprising for example a microcontroller or control logic 12a and probably additionally a UHF unit 12b for sending a UHF signal, and a front-end circuit 14 connected to three LC resonant circuits 16, 18 and 20, which are arranged in a three-dimensional configuration. The UHF transmitter operates at a frequency of about 315 MHz, 434 MHz or 868 MHz. Arrows 22 indicate that the LF transceiver 10b will send an interrogation signal (wake up signal and challenge) to all three LC resonant circuits during an interrogation interval. The interrogation interval is at the same time a capacitor charging phase, as at least one storage capacitor comprised in the transponder will be charged to supply the transponder with energy during the response interval. According to the orientation in space of transponder 12 in relation to interrogator unit 10, one LC resonant circuit will receive the interrogation signal best and the associated receiver channel will be selected. Only the LC resonant circuit which is associated to the selected receiver channel will send a response signal. In FIG. 1 this is LC resonant circuit 16 and the response signal is indicated by an arrow 24. Although FIG. 1 shows both directions for signal transmission, it is to be understood that in a half-duplex transponder receiving and transmitting are separated in time, transponder 12 first receives an interrogator signal 22 and afterwards sends a response 24.
In a keyless entry and keyless go system the low frequency (for example 134.2 kHz) transmission is used for waking and challenging the battery supplied key over a distance of typically several meters. The key uses the UHF transmitter for responding to the R/W-unit (uplink). Some keyless entry and keyless go systems do not require a distance of several meters. For those systems, the configuration shown in FIG. 1 and described hereinabove is too expensive and too complex.